1. Field of the Invention
The present invention relates to a battery condition detection apparatus for determining a charging condition, namely a capacity of an automotive battery accurately.
2. Description of the Related Art
The specific gravity of the battery electrolyte has conventionally been measured by a specific gravity sensor as a means for detecting the battery condition (a residual capacity, etc.). The problem of the above-mentioned conventional means is that if the battery electrode is deteriorated or if a liquid level of the battery drops due to overcharge of the battery, the relationship between the specific gravity and the battery capacity is changed undesirably from the relationship therebetween under a normal condition, thereby making it impossible to measure the battery condition (capacity) accurately.
Another conventional apparatus for detecting the battery condition (a residual capacity, etc.) is well known, as disclosed in JP-A-53-127646, in which internal impedance of a battery is determined from a battery discharge current and a battery voltage, so that the battery charging state is detected on the basis of the internal impedance which is increased with the deterioration of the battery charging condition (a reduction in the residual capacity).
If the battery condition is to be evaluated accurately, however, it is necessary to determine the internal impedance from a battery terminal voltage V.sub.S upon conduction of a predetermined reference discharge current I.sub.S. A starter current under a cranking operation of an engine greatly varies depending on the type and temperature of the engine. The aforementioned conventional apparatus, in which the internal impedance is determined only from a ratio between a starter current happening to flow and the battery terminal voltage at the start of an engine operation, poses a problem such that the battery condition cannot be measured accurately.
Another problem is that, even if it is desired to determine the battery terminal voltage V.sub.S at a moment when the starter current has reached a predetermined reference discharge current level I.sub.S, the starter current does not rise up to the reference discharge current level I.sub.S due to the engine temperature or the like, thus making the measurement impossible.
Further, there was a further problem that the reference discharge current I.sub.S is 150 A to 300 A for an ordinary class of automobiles and 50 A to 100 A for a light-weight class of automobiles, so that it is necessitated to change the setting of the reference discharge current I.sub.S depending on the type of an automobile carrying the battery.
The present invention has been made in order to solve the above-mentioned problems of conventional apparatuses.